SKU (Stock Keeping Unit)
A SKU (Stock Keeping Unit) is a unique, internally assigned identifier for a specific stockable and sellable product variant. It pinpoints every unit of inventory exactly and forms the basis for inventory management, picking and cross-channel selling.
A SKU (Stock Keeping Unit) is a unique identifier a company assigns itself to a specific stockable and sellable product variant. It denotes the smallest unit that can be tracked, counted, priced and sold separately — so not just "T-shirt", but "T-shirt, model X, colour blue, size M". Every distinguishable variant gets its own SKU, so that stock, price and availability can be mapped precisely to a single item.
The term comes from inventory management: the Stock Keeping Unit is the unit that is "kept in stock". Unlike a global product identifier, the SKU is always internal — each company defines its own format and can encode extra information such as category, colour or location into the code. In the ERP or inventory management system, the SKU is the practical key through which the warehouse, purchasing, sales and e-commerce all address the same product variant.
At a glance
- Unique, internally assigned identifier per stockable and sellable product variant
- Smallest distinguishable unit: model + colour + size each get their own SKU
- Internally defined format — as opposed to the globally valid GTIN/EAN
- Basis for inventory management, picking, reordering and reporting
- In multichannel setups, the key to consistent stock across all channels
What is a SKU (Stock Keeping Unit)?
A SKU is an alphanumeric code that represents exactly one sellable product variant. It answers the question "What exactly is sitting in the warehouse and being sold?" at the finest meaningful level. Two products that differ in a sales-relevant attribute — colour, size, pack size, version — are different SKUs, even if they belong to the same overarching product.
The SKU is deliberately internal to the company. Every business sets its own format, often so that attributes can be read straight from the code. This freedom sets the SKU apart from standardised, globally unique identifiers and makes it flexible: it can be tailored precisely to your own assortment, warehouse and channel structure.
Structure and components of a SKU
A typical SKU code is made up of several segments, each encoding an attribute — such as product group, product line, colour, size and possibly location. For example: "SHIRT-BAS-BLU-M" stands for a basic shirt in blue, size M. What matters is brevity, readability and a consistently identical scheme. Leading zeros on purely numeric codes and special characters should be avoided, because they cause errors when exporting to shops, marketplaces or spreadsheets.
SKU vs. item number
In many systems the SKU effectively coincides with the item number in the item master — both identify a variant uniquely. In e-commerce, "SKU" has become the established term, while traditional inventory management systems tend to speak of the item number. What matters is not the name but that there is exactly one unique, stable key per sellable variant.
How SKUs work in the ERP system
In the ERP or inventory management system, the SKU is the key through which a product variant is addressed. It is attached to the relevant record in the item master and links all relevant details there: stock per storage bin, purchase and sales prices, tax rate, dimensions and weight, supplier assignment and the associated GTIN. Every movement — goods receipt, transfer, picking, goods issue — is posted to the SKU, so that stock always remains mapped precisely to a single variant.
Because all modules use the same key, the SKU is the link between purchasing, warehouse, sales and shipping. A reorder proposal refers to a SKU, the goods receipt posts to it, picking removes it, and the invoice sells it. In analysis, the fine granularity enables meaningful metrics: sell-through, inventory turnover and contribution margin can be determined per variant, which is what makes ABC analyses and reorder decisions precise in the first place.
SKU and product variants
Modern systems model a parent product with several variants. The parent product bundles shared attributes (description, images, category), while each product variant carries its own SKU with its own stock and price. This keeps the product catalogue tidy without giving up stock-accurate management at variant level — the prerequisite for a single sold-out size run to be correctly shown as "unavailable".
Why SKUs matter in retail and e-commerce
The value of a clean SKU structure shows in almost every operational process. In the warehouse, a unique SKU speeds up picking, because the right item is identified beyond doubt and mix-ups between similar variants are eliminated. In purchasing, it controls reorder points and replenishment precisely at variant level, instead of only coarsely at product level. And in reporting, it provides the data basis for separating slow movers from fast movers.
The SKU is indispensable especially in multichannel sales. Anyone selling through their own shop, several marketplaces and in store needs a shared internal key to keep stock in sync across channels. Every unit sold reduces the stock of the same SKU — regardless of the channel. Without this unique mapping, you risk overselling because two channels offer the same last unit, or lost revenue from overly cautious safety stock.
Distinctions: SKU vs. GTIN/EAN, item number and barcode
The SKU is often confused with other identifiers, but it serves its own purpose. The most important difference concerns reach: a SKU is internal and company-specific, whereas a GTIN (Global Trade Item Number, usually visible in retail as an EAN) is globally unique and assigned in a standardised way via GS1. The same product carries a different SKU at every retailer, but the same GTIN everywhere.
Internal identifier vs. global standard
The SKU serves internal control: inventory management, picking, reporting and channel reconciliation. The GTIN, by contrast, enables unique identification across company boundaries — for example in a marketplace listing, in the trading partner's point-of-sale system or in electronic data interchange. The two complement each other: in the item master, the SKU is the operational key, the GTIN the external link. The barcode, in turn, is merely the machine-readable representation — usually of the GTIN, rarely of the SKU — and not a numbering scheme of its own.
Building a SKU system: best practices and DACH specifics
A good SKU system is thought through from the start and maintained consistently. Descriptive but short codes with a fixed segment scheme that stays the same across the entire assortment have proven their worth. SKUs should be stable: once assigned, a code is not reused, even when an item is discontinued — otherwise historical analyses become distorted. Special characters, spaces and leading zeros should be avoided, because they break during data import and export to shops, marketplaces and spreadsheets.
For the DACH region, the clean separation of internal SKU and external GTIN is especially relevant: German and Austrian marketplaces as well as many trading partners require a valid GTIN for the listing, but keep their own SKU internally. Since SKUs flow into documents, delivery notes and invoices, they indirectly touch the GoBD requirements for traceability — the unique, immutable mapping of a sold unit must remain reproducible. As the assortment grows, a documented naming convention helps to keep an overview of thousands of variants and avoid duplicates.
Example
Example: a fashion retailer structures 4,000 variants for marketplace selling
A fashion retailer sells the same items through its own shop and two marketplaces. Initially it kept only one product number per model — colours and sizes were not tracked separately. The result: stock could not be managed at variant level, individual sizes were oversold, and no one could say which colour was selling best.
The team introduced a SKU system following the scheme "model-colour-size", for example "JCK-2200-NVY-42". Each of the roughly 4,000 sellable variants received its own SKU with its own stock, linked to the respective GTIN for the marketplace listing. From then on, all channels ran on the same SKU stock in the ERP system. The result: overselling disappeared, picking became unambiguous, and analysis by colour and size drove the next reorder.
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